Thermoplastic materials or so-called "hot melt" materials such as hot melt adhesives are stored in solid form and must be converted to the molten state before they can be supplied to applicators or dispensers. Commercially available thermoplastic melting apparatus generally include a hopper or tank having an open top for receiving solid thermoplastic material, heating elements mounted in the walls or at the bottom of the tank to convert the solid thermoplastic material to the molten state, a reservoir and/or manifold for receiving the molten material and a pump for pumping the molten material from the reservoir and/or manifold to one or more dispensers.
The tank which receives the solid thermoplastic material is normally enclosed within a housing having a cover plate for sealing its open top. The open top of the tank is closed by a lid to prevent contamination of the atmosphere and to avoid exposure of the thermoplastic material to air, which, particularly with hot melt adhesive, can cause oxidation and charring of the material. Both the cover plate and tank lid are removable in order to permit periodic loading of additional solid thermoplastic material into the tank.
Because the contents of the tank are hidden from view by the tank lid and housing cover plate, a level indicator must be provided in order to allow the operator to determine when the tank should be refilled with solid thermoplastic material. In the prior art, level indicators have been mounted within the interior of the tank and are directly exposed to the thermoplastic material therein. These level indicators have proven unreliable, particularly in measuring levels of viscous thermoplastic material such as hot melt adhesive.
One reason for inaccurate measurements by prior art level indicators is they lack the sensitivity to differentiate between the states of the thermoplastic material within the tank, i.e., solid or liquid. This can be a particular problem in melting apparatus having a grid melter or the like at the bottom of the tank wherein the thermoplastic material is in molten or liquid form in the area of the melter but in solid form above the melter. Another problem with prior art level indicators is that they may be sensitive to the temperature within the tank which can vary substantially depending upon the type of thermoplastic material heated therein. Additionally, prior art level indicators may provide an inaccurate reading for thermoplastic materials having different viscosities.